Metallic luster pigments

ABSTRACT

The present invention relates to metallic luster pigments, to a process for production thereof and to the use of such metallic luster pigments.

This application is a 35 U.S.C. 371 national stage filing and claimspriority to PCT Application PCT/EP2014/002073 entitled “METALLIC LUSTREPIGMENTS BASED ON SUBSTRATE FLAKES WITH A THICKNESS OF 1-50 NM,” filedJul. 29, 2014, which claims the benefit of European Application13003870.6 filed Aug. 2, 2013, each of which are incorporated byreference herein in their entirety.

The present invention relates to metallic luster pigments, to a processfor production thereof and to the use of such metallic luster pigments.

Metallic luster pigments or metal effect pigments find wide use in manyfields of industry. They are used, for example, for coloring of paints,printing inks, other inks, plastics, glasses, ceramic products andformulations for decorative cosmetics. Of particular economicsignificance is the use of metallic luster pigments in automotive paintsystems. Because of their uncopyable visual effects, they are also usedin the production of forgeryproof securities and documents such asbanknotes, checks, bank and credit cards, entry tickets and othertickets. They have the particular features of an attractiveangle-dependent color appearance (goniochromism) and a metallic luster.

In the case of standard pigments, a color impression arises merelythrough absorption of particular wavelengths of incident light andscattering reflection. Standard metallic effect pigments reflect theincident light to a high degree and produce a light-dark flop, but nocolor impression. In the case specific metallic luster pigments,however, optical interference effects give rise to a color impression.Metallic luster pigments of this kind, which are generally based on atleast singly coated platelet-shaped substrates, show interferenceeffects as a result of superimposition of various refracted andreflected light rays. White light incident on the flat surface of thecoated substrates is partly reflected on the outer surface of thecoating. The other portion is refracted and is reflected at interfaces,for example between the coating and substrate surface, and refractedagain. The result is therefore superimposition of light rays ofdifferent phases. Interference of the reflected light gives rise to acolor impression. Because of the dependence of the phase differential onthe angle of incidence/observation, the color impression is alsoangle-dependent. This effect of color change between differentreflection angles is referred to as color flop. The phase differentialis affected, inter alia, by the thickness of the coating(s), as a resultof which the color impression that arises can be adjusted via thecoating thickness.

EP-A-0 033 457 describes pigments based on iron oxide-coated aluminumplatelets, which have golden to red hues at the specular angle(reflection angle with highest brightness).

DE 94 00 447 U1 describes luster pigments based on nitrated metal(oxide) platelets which have a high hardness and are suitable for use inpaints and coatings, in the cosmetics sector and for the coloring ofplastics.

WO 2004/113455 describes a method for producing a pigment comprising anSiO_(z) core material and at least one dielectric layer by microwavedeposition of a metal oxide from an aqueous solution.

WO 2005/049739 discloses effect pigments having an aluminum or aluminumalloy core and an aluminum oxide or aluminum oxide/hydroxide-containinglayer that encases the aluminum or aluminum alloy core, obtainable bywet-chemical oxidation of aluminum or aluminum alloy pigments inplatelet form, wherein the content of metallic aluminum in the aluminumor aluminum alloy core is not more than 90% by weight, based on thetotal weight of the pigment.

However, the pigments known from the prior art have considerableshortcomings. For instance, the hiding capacity of known pigments issufficient for certain applications. However, it would be desirable forreasons of efficiency to provide metallic luster pigments having higherhiding capacity. Especially in the case of automotive paint systems,there is a requirement for ever thinner paint layers, which areachievable by means of pigments having higher hiding capacity.Furthermore, metallic luster pigments based on metal oxide-coatedaluminum platelets sometimes have disadvantageous safety properties. Forinstance, pigments of this kind may be inflammable and even be anexplosion hazard. Aluminum reacts particularly violently with iron oxidein particular (thermite reaction). These properties of knownaluminum-based metallic luster pigments restrict operational safety.

It is thus an object of the present invention to provide an inexpensivemetallic luster pigment having a high hiding capacity and a low level offire characteristics from a performance point of view.

The object is achieved by the embodiments designated in the claims.

More particularly, a metallic luster pigment based on coated aluminumsubstrate platelets is provided, wherein the aluminum substrateplatelets have a thickness of 1 to 50 nm, preferably of 1 to 30 nm, areof monolithic structure and have optionally been passivated and areencased by at least one coating B composed of at least one metal oxidehaving a high refractive index of at least 1.9,

wherein the coating B has a thickness of at least 50 nm, and

wherein between the surface of the aluminum substrate platelets and thecoating B there is at least one further coating A which encases thesubstrate platelets and is composed of at least one metal oxide having alow refractive index of at most 1.8, selected from the group consistingof SiO₂, B₂O₃, MnO₂, MgO, GeO₂ and Al₂O₃.

In the context of the present invention, for the sake of simplicity, Si,B and Ge are included among the metals.

The aluminum substrate platelets have an average thickness of at most 50nm, preferably less than 30 nm, more preferably at most 25 nm, forexample at most 20 nm. The average thickness of the aluminum substrateplatelets is at least 1 nm, preferably at least 2.5 nm, more preferablyat least 5 nm, for example at least 10 nm. Preferred ranges for thethickness of the aluminum substrate platelets are 2.5 to 50 nm, 5 to 50nm, 10 to 50 nm; 2.5 to 30 nm, 5 to 30 nm, 10 to 30 nm; 2.5 to 25 nm, 5to 25 nm, 10 to 25 nm, 2.5 to 20 nm, 5 to 20 nm and 10 to 20 nm.Preferably, each substrate platelet has a thickness of maximumhomogeneity. As a result of the production, however, variations inthickness may occur within a platelet. These should preferably be notmore than ±25%, based on the average thickness of the platelets inquestion, more preferably at most ±10%, especially preferably at most±5%. The average thickness is understood here to mean the numericalaverage of maximum and minimum thickness. The minimum and maximum layerthickness are determined by measurement on the basis of a transmissionelectron micrograph (TEM) of a (coated) substrate platelet (cf. FIGS. 2and 3). Since the color of the coated substrate platelets has a lineardependence on the layer thickness, a homogeneous color effect is assuredthrough a precise and homogenized thickness of the uncoated aluminumsubstrate platelets.

Platelets or flakes are understood in the context of the presentinvention to mean those having a thickness/length ratio of at least10:1, preferably higher.

With regard to the variation in layer thickness and the determination ofthe (average) layer thickness, the above also applies analogously to thethicknesses of the coatings A and B and, if present, C.

Where reference is made here to the “thickness” of a coating or analuminum substrate platelet, this means the average thickness, unlessstated otherwise at the point in question.

The aluminum substrate platelets are of monolithic structure.“Monolithic” in this connection means consisting of a singleself-contained unit with no fractures, layering or occlusions, althoughchanges in structure may occur within the substrate platelets (see FIG.2). The aluminum substrate platelets are preferably of homogeneousstructure, meaning that no concentration gradient occurs within theplatelets. More particularly, the aluminum substrate platelets do nothave a layered structure and do not have any particles distributedtherein. More particularly, they do not have a core-shell structure,where the shell consists, for example, of a material suitable for thesubstrate platelets and the core of another material, for example asilicon oxide. By virtue of their simple structure, the substrateplatelets can be produced inexpensively and efficiently. In contrast, amore complex, non-monolithic structure of the substrate plateletsresults in a more complicated, time-consuming and costly productionprocess.

The proportion by mass of the aluminum substrate platelet in the coatedsubstrate platelet is preferably at most 20% by weight, more preferablyat most 15% by weight, for example at most 10% by weight. However, theproportion by mass of the aluminum substrate platelets should not fallbelow 0.1% by weight, preferably not below 0.5% by weight or 1% byweight.

As a result of the low thickness or the low proportion by mass of thealuminum substrate platelets, the metallic luster pigment of theinvention has a particularly high hiding capacity.

The luster pigment of the invention preferably has a total colordifference ΔE of at most 10, more preferably at most 5, especially atmost 3. The measurement of ΔE is effected here to DIN 55987, byapplication of a paint layer comprising the metallic luster pigment ofthe invention in a proportion by weight of 18% by weight (dry weight) toa black surface and a white surface. The layer thickness of the driedcoat of paint is 15 μm. Thereafter, the total color difference ΔEbetween the coats of paint on white and black backgrounds is determined.

In this respect, the present invention, in a further independentembodiment, is also directed to luster pigments having a total colordifference ΔE of at most 10, more preferably at most 5, especially atmost 3.

Regardless of the thickness, the size of the uncoated aluminum substrateplatelets is not critical and can be matched to the particular end use.In general, the platelets have mean greatest diameters of about 2 to 200μm, especially about 5 to 100 μm. The d50 of the uncoated aluminumsubstrate platelets, for use in automotive paints, is preferably 5 to 50μm, more preferably 10 to 30 μm, but may also assume values of about 70μm for other uses, for example as industrial paint.

In this document, the d50, unless stated otherwise, is determined with aSympatec Helos instrument with Quixel wet dispersion. The sample isprepared by pre-dispersing the sample to be analyzed in isopropanol fora period of 3 minutes.

The coated aluminum substrate platelets preferably have a thickness of70 to 500 nm, more preferably 100 to 400 nm, especially preferably 150to 320 nm, for example 180 to 290 nm. Because of the low thickness ofthe substrate platelets, the metallic luster pigment of the inventionhas a particularly high hiding capacity. The low thickness of the coatedaluminum substrate platelets is especially achieved by virtue of thethickness of the uncoated substrate platelets being low, but also byvirtue of the thicknesses of the coatings A and, if present, C beingadjusted to a minimum value. Since the thickness of coating B determinesthe color impression of the metallic luster pigment, there is no roomfor maneuver in this regard in the case of a fixed desired color effect.

It has been assumed to date that exclusively non-transparent (opaque)materials are suitable as substrate platelets. In addition, it has beenassumed that uncoated substrate platelets must not go below a certainthickness, in order to avoid (partial) transparency thereof, which wouldlead, according to this assumption, to a significantly lowered hidingcapacity of the resulting luster pigment.

However, it has been found that, surprisingly, it is possible to use(partly or completely transparent) aluminum substrate platelets having alayer thickness of at most 50 nm, preferably at most or less than 30 nm,to produce metallic luster pigments having a much higher hiding capacitythan standard metallic luster pigments. The reason for this is probablythat the low thickness of the coated aluminum substrate plateletsachieves a higher area coverage of the metallic luster pigment. Sincethe coated substrate platelets are thin, the same mass of pigment cancover a higher area. This advantageous effect more than compensates forthe higher transparency of thin, completely or partly transparentsubstrate platelets, so as ultimately to achieve a higher hidingcapacity compared to metallic luster pigments having thick substrateplatelets.

Aluminum platelets can be produced, inter alia, by punching out ofaluminum foil or by standard grinding and jetting techniques. Forexample, aluminum platelets are obtainable from the Hall process, a wetgrinding process.

The aluminum platelets may take various forms. Substrate platelets usedmay, for example, be lamellar and lenticular metal platelets or elsewhat are called vacuum metallized pigments (VMP). Lamellar metalplatelets feature an irregularly structured edge and, because of theirappearance, are also referred to as “cornflakes”. Lenticular metalplatelets have an essentially regular round edge and, because of theirappearance, are also referred to as “silver dollars”. Because of theirirregular structure, metallic luster pigments based on lamellar metalplatelets produce a higher proportion of scattered light than lenticularmetal platelets, whereas the proportion of reflective light ispredominant in the case of the latter.

The aluminum platelets may be passivated, for example by eloxation(oxide layer) or chromation.

VMPs may be obtained by the release of aluminum from metallized foils.They feature a particularly low thickness of the substrate platelets inthe range from 5 to 50 nm, preferably up to or less than 30 nm, and aparticularly smooth surface having elevated reflectivity. In the contextof the present invention, Al-VMPs are preferred.

The metallic luster pigment of the invention may be a luster pigmenteither of the leafing type or of the non-leafing type. The metallicluster pigment is preferably a luster pigment of the non-leafing type.

According to the invention, the coated aluminum substrate platelets areencased by at least one coating B of a metal oxide having a highrefractive index with a coating thickness of at least 50 nm. Between thecoating B and the substrate surface, the coated substrate platelets haveat coating A. The substrate platelets may have a further coating Cdifferent than the layer B beneath.

In general, coating of part of the surface of the coated substrateplatelets is sufficient to obtain a luster pigment. For example, it ispossible for only the upper and/or lower side of the platelets to becoated, leaving the side face(s) as they are. According to theinvention, however, the entire surface of the optionally passivatedsubstrate platelets, including the side faces, is covered by coating B.The substrate platelets are thus covered completely by coating B. Thisimproves the optical properties of the pigment of the invention andincreases the mechanical and chemical durability of the coated substrateplatelets. The above also applies to the layer A and preferably also tothe layer C, if present.

Even though a plurality of coatings A, B and/or C may be present in eachcase, the coated substrate platelets preferably each have just onecoating A, B and, if present, C.

The coating B is formed from at least one metal oxide of high refractiveindex. Preferably, the coating B comprises at least 95% by weight, morepreferably at least 99% by weight, for example about 100% by weight, ofat least one metal oxide of high refractive index.

The coating B has a thickness of at least 50 nm. Preferably, thethickness of coating B is not more than 400 nm, more preferably at most300 nm.

The ratio of the thickness of coating B to the thickness of the uncoatedaluminum substrate platelets is preferably at least 2, for example 4, 8or 10. In principle, it is not necessary to observe any upper limit forthis ratio, but for practical reasons it should be at most 1000,preferably at most 500. The average thickness of a coating or asubstrate platelet is determined from the arithmetic mean of the maximumand minimum thickness of the coating/substrate platelet.

Where reference is made here to “substrate platelets” without anydistinction as to whether they are coated or not, this refers touncoated substrate platelets unless stipulated otherwise at the point inquestion.

According to the invention, between the surface of the aluminumsubstrate platelets and coating B, there is a further coating A composedof at least one metal oxide having a low refractive index of at most1.8, selected from the group consisting of SiO₂, B₂O₃, MnO₂, MgO, GeO₂and Al₂O₃. Preferably, coating A comprises at least 95% by weight, morepreferably at least 99% by weight, for example about 100% by weight, ofsuch a metal oxide of low refractive index

Occasionally, the metal oxides which can be used for coatings A, B and Chave a certain proportion of secondary constituents and/or impurities.Typical secondary constituents of metal oxides especially include metalhydroxides. For example, a coating of iron oxide may contain a certainproportion of iron hydroxide.

The terms “high refractive index” and “low refractive index” here referrespectively to materials having high and low refractive indices.Materials having a high refractive index have a refractive index of atleast 1.9, preferably at least 2.0 and more preferably at least 2.4.Materials of low refractive index have a refractive index of at most1.8, preferably at most 1.6.

The term “essentially”, when applied to a constituent of a composition,means that the composition is formed from the constituent referred to anextent of at least 95% by weight, preferably to an extent of at least99% by weight, especially preferably to an extent of at least 99% byweight, for example to an extent of about 100% by weight.

Metal oxides of high refractive index that are suitable for coating Bare preferably selectively light-absorbing (i.e. colored) metal oxidessuch as, more particularly, iron(III) oxide (α- and γ-Fe₂O₃, red),cobalt(II) oxide (blue), chromium(III) oxide (green), titanium(III)oxide (blue, typically present in a mixture with titanium oxynitridesand titanium nitrides) and vanadium(V) oxide (orange), and mixturesthereof. Also suitable are colorless oxides of high refractive index,such as titanium dioxide and/or zirconium oxide.

Coating B may contain a selectively absorbing dye, preferably 0.001% to5% by weight, more preferably 0.01% to 1% by weight. Suitable organicand inorganic dyes are those which can be incorporated stably into ametal oxide coating.

Among the metal oxides of low refractive index that are envisaged inaccordance with the invention for the coating A, silicon dioxide ispreferred. Coating A preferably has a thickness of 1 to 100 nm, morepreferably 5 to 50 nm, especially preferably 5 to 20 nm. Preferably, thedistance between the surface of the substrate platelets and the innersurface of the coating B is at most 100 nm, more preferably at most 50nm, especially preferably at most 20 nm. By virtue of the thickness ofcoating A/the distance between the surface of the substrate plateletsand coating B being within the above-specified range, it is possible toensure that the coated substrate platelets of the metallic lusterpigment of the invention have a high hiding capacity and hence a minimumΔE value.

In a preferred embodiment, the substrate platelets have a furthercoating C of a metal oxide (hydrate) different than the coating Bbeneath. Suitable metal oxides are, for example, silicon (di)oxide,silicon oxide hydrate, aluminum oxide, aluminum oxide hydrate, zincoxide, tin oxide, titanium dioxide, zirconium oxide, iron(III) oxide andchromium(III) oxide. Preference is given to silicon dioxide.

The coating C preferably has a thickness of 10 to 500 nm, morepreferably 50 to 300 nm. Through the provision of coating C, for examplebased on TiO₂, it is possible to achieve better interference, whileensuring a high hiding capacity.

In the metallic luster pigment of the invention and the coated substrateplatelets, the quantitative ratio α of oxygen not bound to aluminumrelative to aluminum is preferably at least 3, more preferably at least4, especially preferably at least 5. If a is at least 3, this avoids thepresence in the coated substrate platelets of a quantitative ratio ofoxygen not bound to aluminum relative to aluminum in a stoichiometricratio of 3/2 (mol/mol). A mixture of aluminum and oxygen compounds,especially Fe₂O₃, in which the quantitative ratio α is in the region of3/2 can react in a highly exothermic manner because of the highoxophilicity of aluminum metal, under some circumstances explosively(aluminothermism, thermite reaction). Therefore, a mixture having α inthe region of 3/2 may constitute a safety hazard. However, thereactivity of such a mixture can be lowered by adjusting the ratio α toa value much greater or much smaller than 3/2 (see FIG. 1).

In order to achieve a low value of α, the aluminum content would have tobe set at a high level. This would be associated with the disadvantagethat the thickness of the substrate platelets of aluminum would have tobe set at a high level, the effect of which would be that the hidingcapacity of such a luster pigment would be greatly lowered.

Thus, the quantitative ratio α is preferably set to a value above 3/2,namely ≥3. As a result, the coated substrate platelets and the metallicluster pigment of the invention exhibit at least only very lowreactivity, if any, with simultaneously high hiding capacity.

The reactivity of the luster pigment is also suppressed or negligiblylow when the proportion by mass of iron(III) oxide, if provided ascoating B, either alone or in combination with a layer C selected fromTiO₂, SnO₂ and Al₂O₃, in the coated substrate platelets is high,preferably at least 65% by weight, more preferably at least 70% byweight, especially preferably at least 75% by weight. However, thecontent of iron(III) oxide should not be more than 99% by weight,preferably not more than 97% by weight.

In addition, the reactivity of the luster pigment is also suppressed ornegligibly low when the content of oxygen not bound to aluminum in themetallic luster pigment is at least 50 mol %, preferably at least 52.5mol %, more preferably at least 55 mol %, for example at least 57 mol %.However, the quantitative proportion of oxygen not bound to aluminumshould not be more than 59 mol %.

In a further independent aspect, the present invention relates to ametallic luster pigment based on coated aluminum substrate platelets,wherein the aluminum platelets are in monolithic form and are encased byat least one coating B of iron(III) oxide which is optionally encased inturn by a layer C selected from TiO₂, SnO₂ and Al₂O₃, especially TiO₂,and wherein the proportion by mass of iron(III) oxide in the coatedaluminum substrate platelets, either alone or in combination with TiO₂,SnO₂ or Al₂O₃ from layer C, if present, is at least 65% by weight, andbetween the surface of the aluminum platelets and coating B there is afurther coating A of at least one metal oxide having a low refractiveindex of at most 1.8, selected from the group consisting of SiO₂, B₂O₃,MnO₂, MgO, GeO₂ and Al₂O₃.

Unless stated otherwise hereinafter, the statements already made aboveapply to the metallic luster pigment according to this aspect.

A stoichiometric mixture of aluminum and Fe₂O₃ (2Al+Fe₂O₃→2Fe+Al₂O₃) canreact in a highly exothermic manner because of the high oxophilicity ofaluminum metal, under some circumstances explosively (aluminothermism,thermite reaction). Therefore, such a mixture may constitute a safetyhazard. However, the reactivity of a mixture of aluminum and iron(III)oxide can be lowered by adjusting the proportion by mass of iron(III)oxide to a very high or very small value (see FIG. 1).

In this aspect of the invention, the proportion by mass of iron(III)oxide, either alone or in combination with TiO₂, SnO₂ or Al₂O₃ fromlayer C, if present, is adjusted preferably to a value of ≥65% byweight, based on the total weight of the coated substrate platelets. Asa result, the coated substrate platelets or the metallic luster pigmentof the invention exhibit(s) at least only very low reactivity, if any.

The proportion by mass of iron(III) oxide in the coated substrateplatelets, either alone or in combination with TiO₂, SnO₂ or Al₂O₃ fromlayer C, if present, is preferably at least 70% by weight, morepreferably at least 75% by weight. However, the content of iron(III)oxide should not be more than 99% by weight, preferably not more than97% by weight.

The reactivity of the coated substrate platelets, in a preferredembodiment of the present invention, is lowered by virtue of theproportion by mass of aluminum metal in the coated substrate plateletsbeing preferably at most 20% by weight, particularly preferably at most15% by weight, more preferably at most 12% by weight, for example atmost 10% by weight. However, the proportion by mass of aluminum metalshould not fall below 0.1% by weight, preferably not below 0.5% byweight or 1% by weight.

Elemental aluminum reacts exothermically not just with iron oxide butalso with a multitude of other oxygen compounds in which oxygen is notbound to aluminum (aluminothermism). Therefore, in the metallic lusterpigment of the invention and the coated aluminum platelets, thequantitative ratio α of oxygen not bound to aluminum relative toaluminum is preferably at least 3, more preferably at least 4,especially preferably at least 5. When α is at least 3 this avoids thepresence in the coated aluminum platelets of a quantitative ratio ofoxygen not bound to aluminum relative to aluminum in a stoichiometricratio of 3/2 (mol/mol). A mixture of aluminum and oxygen compounds (forexample Fe₂O₃) in which the quantitative ratio α is in the region of 3/2can react in a highly exothermic manner, explosively under somecircumstances (aluminothermism). Therefore, a mixture with a in theregion of 3/2 may constitute a safety hazard. However, the reactivity ofsuch a mixture can be lowered by adjusting the ratio α to a value muchgreater or much smaller than 3/2 (see FIG. 1 again).

The quantitative ratio α is preferably adjusted to the above-specifiedvalue above 3/2, namely ≥3. As a result, the coated aluminum plateletsand the metallic luster pigment of the invention show at least only verylow reactivity, if any.

In this aspect of the metallic luster pigment of the invention, noparticular demand is made on the thickness of the aluminum platelets,provided that the proportion by mass of iron(III) oxide and preferablyalso the quantitative ratio α are as specified above. Preferably, thealuminum platelets have a thickness of 1 nm to 20 μm.

In one embodiment, the thickness of the aluminum platelets is preferablyin the range from 1 to 50 nm, more preferably 1 to 30 nm, especiallypreferably 1 to 25 nm. In this case, the thickness of the iron oxidecoating is at least 100 nm and is preferably in the range from 100 to300 nm.

In another embodiment, the thickness of the aluminum platelets ispreferably in the range from 100 to 1500 nm. In this case, the coating Bpreferably has a thickness of 300 to 3500 nm. More preferably, the layerthickness of the coating B is from 400 to 800 nm. If the thickness ofthe aluminum platelets and coating B is set within this range, it ispossible to provide metallic effect pigments having particularly highsparkle.

The process of the invention for producing the metallic luster pigmentcomprises the steps of providing optionally passivated aluminumsubstrate platelets, and of coating the aluminum substrate platelets byhydrolytic decomposition of one or more organic metal compounds and/orby precipitation of one or more dissolved metal salts.

To produce coating A, appropriately organic metal compounds (preferablyorganic silicon compounds) in which the organic radicals are bonded tothe metals by oxygen atoms are hydrolyzed in the presence of thesubstrate platelets and an organic solvent in which the metal compoundsare soluble. A multitude of organic solvents are suitable for thispurpose, preference being given to isopropanol. In the case of SiO₂, thecoating A can also be produced in an aqueous medium.

Preferred examples of the organic metal compounds are theacetylacetonates and especially alkoxides, in particularC₁-C₄-alkoxides, e.g. aluminum triisopropoxide and tetraethoxysilane(tetraethyl orthosilicate, TEOS).

The hydrolysis is preferably conducted in the presence of a base or anacid as catalyst. Suitable bases for the purpose are, for example, notonly aqueous alkalis such as sodium hydroxide solution, but especiallyaqueous ammonia solutions.

Suitable acidic catalysts are, for example, phosphoric acid and organicacids such as acetic acid and oxalic acid.

Water has to be present at least in the amount required instoichiometric terms for the hydrolysis, but preference is given to 2 to100 times the amount, especially 5 to 20 times the amount.

Based on the amount of water used, generally 3% to 40% by volume,preferably 5% to 30% by volume, of a 25% by weight aqueous ammoniasolution is added.

For the thermal regime, it has been found to be advantageous to heat thereaction mixture to reflux temperature stepwise within 10 to 48 h. Whenisopropanol is used as solvent, the mixture is stirred, for example,preferably first at 40° C. for 4 to 20 h, then at 60° C. for 4 to 20 hand finally at 80° C. for 2 to 8 h.

In terms of process technology, the coating of substrate platelets witha coating A is appropriately effected as follows:

Aluminum substrate platelets, organic solvent, water and catalyst (acidor preferably base, especially, for example, an aqueous ammoniasolution) are initially charged, and then the metal compound to behydrolyzed is added as a pure substance or in dissolved form, forexample as a 30% to 70%, preferably 40% to 60%, by volume solution inthe organic solvent. If the metal compound is added in one step, thesuspension is then subsequently heated while stirring as describedabove. Alternatively, the metal compound can be metered in continuouslyat elevated temperature, in which case water and ammonia may beinitially charged or likewise metered in continuously. After the coatinghas ended, the reaction mixture is cooled back down to room temperature.

In order to prevent agglomerate formation during the coating operation,the suspension can be subjected to significant mechanical stress, suchas pumping, vigorous stirring or the action of ultrasound.

Optionally, the coating step can be repeated once or more than once.Should the mother liquor have a milky opaque appearance, it is advisableto exchange it prior to a further coating operation.

The aluminum substrate platelets ensheathed with the coating A can beisolated in a simple manner by filtration, washing with organic solvent,preferably the alcohols used as solvent, and then drying (typically at20 to 200° C. for 2 to 24 h).

For application of the metal oxide layers (B), it is possible to applyα-iron oxide and chromium oxide layers by hydrolytic decomposition ofiron(III) salts such as iron(III) chloride and sulfate or chromium(III)chloride, followed by conversion of the hydroxide-containing layersformed to the oxide layers by heat treatment. The heat treatment ispreferably effected at a temperature of 250 to 550° C. for a period of 5to 60 minutes, preferably 350 to 450° C. for a period of 10 to 30minutes. It is likewise also possible to achieve a titanium(III) oxidecoating by hydrolysis of titanium tetrachloride and subsequent reductionof the titanium dioxide formed with gaseous ammonia.

If a coating C is desired, this can be applied as described for coatingsA and B.

With the aid of the production process according to the invention, it ispossible to produce the coated substrate platelets reproducibly in largevolumes in a simple manner. Fully encased pigment particles having ahigh quality of the individual coatings (homogeneous, film-like) areobtained.

The present invention further relates, in a further aspect, to the useof the above-described metallic luster pigments for coloring of paints,printing inks, other inks, plastics, glasses, ceramic products andformulations for decorative cosmetics.

The luster pigments of the invention are advantageously suitable formany purposes, such as for coloring of plastics, glasses, ceramicproducts, formulations for decorative cosmetics, and especially of inks,printing inks and security printing inks, and in particular of paints,for example for the automotive industry.

For these end uses, the pigments of the invention can alsoadvantageously be used in a blend with transparent and hiding white,chromatic and black pigments, and also conventional luster pigmentsbased on metal oxide-coated mica and metal pigments and iron oxides inplatelet form.

The metal luster pigments of the invention can be producedinexpensively. They have an exceptionally high hiding capacity and thusoffer a variety of advantages for use thereof, for example as a paint inthe automobile and motor vehicle industry. The metallic luster pigmentsaccording to the present invention additionally have low flammability.They therefore meet strict fire protection regulations and safetyrequirements.

FIGS. 1 and 2 show results from combustion tests on various coatedaluminum platelets having a different proportion by weight of aluminum(FIG. 1) and iron(III) oxide (FIG. 2). The performance rating for thefire characteristics (y axis) as a function of the proportion ofaluminum (FIG. 1) or iron(III) oxide (FIG. 2) in the coated substrateplatelets is determined by assessing the behavior of the sample in thecombustion test described hereinafter.

FIG. 3 shows a coated aluminum platelet of the invention. The aluminumplatelet has a very homogeneous thickness and is encased by an SiO₂layer (coating A, light) and an iron oxide layer (coating B, dark).

FIG. 4 shows a coated aluminum platelet of the invention having analuminum core, SiO₂ layer (coating A, light) and iron oxide layer(coating B, dark).

The examples which follow serve to further illustrate the presentinvention, without being restricted thereto.

EXAMPLE 1 (THIN ALUMINUM PLATELETS WITH THICK IRON OXIDE COATING)

First of all, 50 g of Al platelets (thickness between 20 nm and 30 nm,d50=12 μm) were coated with 10 g of SiO₂ by means of a sol-gel methodusing tetraethyl orthosilicate (TEOS). In a round bottom flask withreflux condenser and stirrer, these Al platelets were admixed with 500mL of deionized water and heated to 75° C. while stirring. The pH wasadjusted to a value of 3.2 by adding a 10% NaOH solution. 1016 g of a20% FeCl₃ solution were added to the reaction mixture, in the course ofwhich the pH was kept essentially constant at 3.2 by simultaneousaddition of a 10% NaOH solution. On completion of addition of the FeCl₃solution, the mixture was stirred for a further 15 minutes, in order toassure complete precipitation. The pH was then increased to a value of7.0 by dropwise addition of a 10% NaOH solution over a period of 30minutes. After stirring for a further 30 minutes, the coated pigment wasseparated from the upernatant reaction solution by filtering and washeduntil it was free of salts. The resultant coated aluminum platelets weredried at 250° C. for 215 minutes and sieved with a sieve (mesh size 25μm). The resultant product was subjected to an assessment of its colorproperties and to a fire test as described below.

EXAMPLE 2 (THIN ALUMINUM PLATELETS WITH THIN IRON OXIDE COATING)

In this example, analogously to the method of example 1, coated aluminumplatelets were produced, with the difference that, rather than 1016 g,only 102 g of the 20% FeCl₃ solution were used. The resultant productwas subjected to an assessment of its color properties and to a firetest as described below.

EXAMPLE 3 (THIN ALUMINUM PLATELETS WITH THICK IRON OXIDE COATING ANDTITANIUM OXIDE COATING)

This example was conducted analogously to example 1 up to and includingthe stirring for fifteen minutes after the addition of the FeCl₃solution had ended. Thereafter, the pH was adjusted to 2.0 by adding 10%HCl solution. 412 g of a 30% TiCl₄ solution were added to the reactionmixture, in the course of which the pH was kept essentially constant at2.0 by simultaneously adding a 10% NaOH solution. On completion ofaddition of the TiCl₄ solution, the mixture was stirred for a further 15minutes, in order to assure complete precipitation. The pH was thenincreased to a value of 7.0 by dropwise addition of a 10% NaOH solutionover a period of 30 minutes. After stirring for a further 30 minutes,the coated pigment was separated from the supernatant reaction solutionby filtering and washed until it was free of salts. The resultant coatedaluminum platelets were dried at 250° C. and sieved with a sieve (meshsize 25 μm). The resultant product was subjected to an assessment of itscolor properties and to a fire test as described below.

COMPARATIVE EXAMPLE (THICK ALUMINUM PLATELETS WITH IRON OXIDE COATING)

First of all, 50 g of Al platelets (thickness between 150 nm and 300 nm,d50=18 μm) were coated with 8.8 g of SiO₂ by means of a sol-gel methodusing tetraethyl orthosilicate (TEOS). In a round bottom flask withreflux condenser and stirrer, these Al platelets were admixed with 500mL of deionized water and heated to 75° C. while stirring. The pH wasadjusted to a value of 3.2 by adding a 10% NaOH solution. 660 g of a 20%FeCl₃ solution were added to the reaction mixture, in the course ofwhich the pH was kept essentially constant at 3.2 by simultaneouslyadding a 10% NaOH solution. On completion of addition of the FeCl₃solution, the mixture was stirred for a further 15 minutes in order toassure complete precipitation. The pH was then increased to a value of7.0 by dropwise addition of a 10% NaOH solution over a period of 30minutes. After stirring for a further 30 minutes, the coated pigment wasseparated from the supernatant reaction solution by filtration andwashed until it was free of salts. The resultant coated aluminumplatelets were dried at 250° C. and sieved with a sieve (mesh size 40μm). The resultant product was subjected to an assessment of its colorproperties and to a fire test as described below.

Combustion Test

20 g in each case of the pigments produced were mixed thoroughly with13.3 g of white spirit. 2 g of this mixture were applied to a glassplate and set on fire. This combustion test is recorded as a video. Thefire characteristics are rated on a scale from 0 to 5, 0 meaning thatthere is merely gentle burnoff of the white spirit without occurrence ofany further reaction.

-   1: isolated sparks during the solvent fire-   2: slight evolution of sparks during the solvent fire-   3: moderate evolution of sparks during the solvent fire-   4: significant evolution of sparks and small explosions or crackling    during the solvent fire, glowing of the sample after the solvent has    burnt off-   5: significant evolution of sparks and explosions or crackling    during the solvent fire, and complete conversion of the sample after    the solvent has burnt off

To measure the total color difference ΔE, a paint layer which comprisedthe metallic luster pigment of the invention to be examined in aproportion by mass of 18% by weight (dry weight) was applied to a blacksurface and to a white surface. The layer thickness of the dried coat ofpaint was 15 μm. Thereafter, the total color difference ΔE between thecoats of paint on white and black backgrounds was determined. Theresults of the measurements are shown in table 1.

Table 1 shows results for various metallic luster pigments. The uncoatedsubstrate platelets consist of aluminum metal. Test numbers 1 to 12 wereproduced in accordance with the methods from examples 1, 2 and 3 withthe necessary modifications for establishment of the individuallyspecific parameters (for example thickness of coatings A, B and, ifpresent, C). Test numbers 5 to 7 and 12 are examples of the presentinvention; tests 1 to 4 and 8 to 11 are comparative examples. Inaddition to test numbers 1 to 12, table 1 shows values for thecommercially available products Paliochrom L2800 (from BASF) and MeoxalOrange (from Merck) as comparative examples.

It is apparent from table 1 and FIG. 1 that, with a content of aluminummetal in the coated substrate platelets of, in particular, equal to orless than 20% by weight, it is possible to provide a pigment which isnoncombustible and is not an explosion hazard. This corresponds to aresult of 1 or 0 in the combustion test.

It is apparent from table 1 and FIG. 2 that, with an Fe₂O₃ content inthe coated substrate platelets of, in particular, equal to or greaterthan 65% by weight, it is possible to provide a pigment which isnoncombustible and is not an explosion hazard. This corresponds to aresult of 1 or 0 in the combustion test.

In addition, table 1 shows that the metallic luster pigments of theinvention have a particularly small color difference ΔE and hence aparticularly high hiding capacity. The proportions reported in the tableare based on % by weight.

TABLE 1 Al SiO₂ Fe₂O₃ TiO₂ Performance rating for Use of substrate ofthe No. content content content content fire characteristics ΔEinvention 1 37 14 39 0 5 0.8 yes 2 34 21 35 0 5 0.6 yes 3 26 30 30 0 50.8 yes 4 22 25 45 0 4 0.2 yes 5 11 13 68 0 1 0.7 yes 6 6 8 73 0 0 0.4yes 7 4 5 72 0 0 3.3 yes 8 4 6 45 30 0 15.4 yes 9 37 7 49 0 5 0.4 yes 1051 6 32 0 4 5.5 no 11 64 4 20 0 2 11.2 no 12 8 3 76 0 0 0.8 yesPaliocrom 69 1 26.6 nd 3 12.0 no L2800 Meoxal Orange 27 11 50.9 nd 525.0 no

The invention claimed is:
 1. A metallic luster pigment based on coatedaluminum substrate platelets, wherein the aluminum substrate plateletshave a thickness of 5 to 30 nm, are of monolithic structure and haveoptionally been passivated and are encased by one coating B, wherein thecoating B has a thickness of at least 50 nm and is formed essentiallyfrom iron(III) oxide, and wherein between the surface of the aluminumsubstrate platelets and the coating B there is at least one furthercoating A which encases the substrate platelets and is composed of atleast one metal oxide having a low refractive index of at most 1.8,selected from the group consisting of SiO₂, B₂O₃, MnO₂, MgO, GeO₂ andAl₂O₃, wherein the substrate platelets have a further coating C composedof at least one metal oxide (hydrate) selected from silicon (di)oxide,silicon oxide hydrate, aluminum oxide, aluminum oxide hydrate, zincoxide, tin oxide, titanium dioxide, zirconium oxide, and chromium(III)oxide.
 2. The metallic luster pigment as claimed in claim 1, wherein thecoating A is composed of SiO₂.
 3. The metallic luster pigment as claimedin claim 1, wherein the coating A has a thickness of 1 to 100 nm.
 4. Themetallic luster pigment as claimed in claim 1, wherein layer A is formedfrom SiO₂ in a thickness of 5 to 50 nm and layer B from Fe₂O₃ in athickness of 50 to 300 nm.
 5. A process for producing metallic lusterpigments as claimed in claim 1, comprising the steps of: providingoptionally passivated aluminum substrate platelets, coating the aluminumsubstrate platelets by hydrolytic decomposition of one or more organicmetal compounds and/or by precipitation of one or more dissolved metalsalts.
 6. A composition comprising: a metallic luster pigment based oncoated aluminum substrate platelets, wherein the aluminum substrateplatelets have a thickness of 5 to 30 nm, are of monolithic structureand have optionally been passivated and are encased by one coating B,wherein the coating B has a thickness of at least 50 nm and is formedessentially from iron(III) oxide and wherein between the surface of thealuminum substrate platelets and the coating B there is at least onefurther coating A which encases the substrate platelets and is composedof at least one metal oxide having a low refractive index of at most1.8, selected from the group consisting of SiO₂, B₂O₃, MnO₂, MgO, GeO₂and Al₂O₃, wherein the substrate platelets have a further coating Ccomposed of at least one metal oxide (hydrate) selected from silicon(di)oxide, silicon oxide hydrate, aluminum oxide, aluminum oxidehydrate, zinc oxide, tin oxide, titanium dioxide, zirconium oxide, andchromium(III) oxide.
 7. The composition as claimed in claim 6, whereinthe coating A is composed of SiO₂.
 8. The metallic luster pigment asclaimed in claim 6, wherein the coating A has a thickness of 1 to 100nm.
 9. The composition as claimed in claim 6, layer A is formed fromSiO₂ in a thickness of 5 to 50 nm and layer B from Fe₂O₃ in a thicknessof 50 to 300 nm.
 10. The composition of claim 6, wherein the compositionis a paint, a printing ink, a plastics, a glass, a ceramic product, or adecorative cosmetic.